A pneumatically driven fuel pump of this kind is disclosed in U.S. Pat. No. 4,936,267. The pump membrane partitions the work chamber from the pump chamber so that the membrane is actuated during the suction stroke directly by the crankcase pressure in the manner of a pump stroke. Fuel is drawn into the pump chamber by suction via the suction valve configured as a check valve and is pumped through the pressure valve into a pressure line. The pressure valve is configured as a check valve. The pressure valve lies diametrically opposite the suction valve referred to the pump chamber so that liquid columns of considerable length have to be accelerated during the suction stroke as well as during the pump stroke. This leads to pumping problems especially at high rpms whereby a deficiency of fuel can occur at the injection pump fed by the pressure line or at a carburetor and this can lead to disturbances.
It has been observed that the rapid change from overpressure to a considerable underpressure at high rpm can cause a pulse-like lifting of the pump membrane whereby vapor bubbles can form which significantly reduce the pumping capacity.